Filter elements that use meltblown fibers to capture airborne particulates have been known and used for many years. Such fiber-containing filter elements are commonly used in respirators to supply clean filtered air to the wearer, as described in, for example, U.S. Pat. No. 5,656,368 to Braun et al., U.S. Pat. No. 5,307,796 to Kronzer et al., and U.S. Pat. No. 4,729,371 to Krueger et al. An electric charge is typically placed on the fibers to improve their capturing efficiency. In 1980, Kubik et al. described a method for introducing a persistent electric charge into meltblown fibers during fiber formation (see U.S. Pat. No. 4,215,682). After the Kubik et al. development, other charging techniques were developed for making electret fibers, as described in, for example, U.S. Pat. Nos. 4,588,537 to Klaase et al., 5,227,172 to Deeds et al., and 5,496,507 to Angadjivand et al.
So-called staple fibers have also been added to meltblown nonwoven webs. For example U.S. Pat. No. 6,827,764 to Springett et al. describes a filter element that comprises a porous molded web that contains thermally bonded staple fibers and non-thermally bonded electrically charged meltblown fibers.
When used as a filter element, webs of electrically-charged fibers are often supported by another structure. For example, in respirators, the filter webs may be supported by a permanently-molded shaping layer. The filter web is positioned over the shaping layer and is secured to it to assume its molded configuration. Examples of patents that disclose the use of a separate shaping layer to support a meltblown fiber-containing filter layer in a respirator include U.S. Pat. Nos. 4,536,440 to Berg, 4,807,619 to Dyrud et al., 4,850,347 to Skov, and 6,923,182 to Angadjivand et al.
In addition to these approaches, filtering webs of polymeric fiber also have been retained in a shaped configuration through use of fabrics, spaced bands, filaments, or fibers, such as described in U.S. Pat. No. 5,656,368 to Braun et al., for example.